im practicing to take the Registory of guitar tutors grade 6 exam, has anyone got any advice on what to practice and what are the hardest parts? a main section of the exam is lead playing, the hardest part of this is identifying what scale to play when it can be dorian or mixolydian major or natural minor etc, could someone give me a good way of practicing identifying the scales to play?

You need to look at the overall chord progression and find the key = tonic chord. That way you will know what scale to use (it's the same as the tonic chord, for example if the progression was C-F-G, the tonic chord would be C and the scale would be C major).

i've doen preliminary grade to grade 5, my music hteory does lack but i've gotten away with it up untill now, this grade is really theory based and belive me it's not easy, rgt is the gold standard in united kingdom and grade 6 gets you points to university